In general, an in-line screw type injection molding machine has a heating cylinder and a screw disposed within the heating cylinder. During the metering process, the screw is rotated by driving it so that a feedstock resin is charged into the heating cylinder, and the feedstock resin is plasticated so that a molten resin is stored in the heating cylinder. Thereafter, the screw is driven and advanced to inject the molten resin into the mold cavity so that a molded object will be produced.
As described above, in the metering process, the screw is driven and rotated so that a molten resin will be stored in the cylinder. During this rotation, the screw is caused to gradually retract by the molten resin. Thus, the amount of the molten resin can be determined on the basis of the position to which the screw has retracted.
When a predetermined amount of a molten resin is to be stored in a heating cylinder, a conventional in-line screw type injection molding machine adopts a rotational speed control method, such as the following: A limit switch detects that the screw has reached a previously-set retracting position and, upon the detection, the rotation driving of the screw is stopped so that a previously-set amount of molten resin is stored in the heating cylinder.
If, as in the conventional machine, the driving of the screw is stopped when the screw has reached a previously-set retracting position, the following problem arises: Due to the inertia, etc. of the screw, the screw may move beyond the previously-set retracting position, thereby making it impossible to accurately meter the molten resin. As a result, the quality of the molded object may be degraded.